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12. Heat-controlled size evolution of dodecanethiol-protected Au nanoparticles in the solid state
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A simple method to control the size of Au nanoparticles via heat treatment. The Au nanoparticles grew to between 3 and 10 nm in size at 150 to 250 degrees C.

Contributors: Teranishi T, Hasegawa S, Shimizu T

Manufactured Nanomaterial or Structure: Gold nanoparticles
Properties: Particle size ranges from 3.4 to 6.8 nm. Au content of particles from 71.9 to 94.7 wt.-% with an increase in heat-treatment temperature.
TEM image of DT-Au nanoparticles heat treated at 230 C. (Image  from Teranishi, et. al. 2001. Advanced Materials 13 (22): 1699-1701. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.) Magnify
TEM image of DT-Au nanoparticles heat treated at 230 C. (Image  from Teranishi, et. al. 2001. Advanced Materials 13 (22): 1699-1701. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

TEM image of DT-Au nanoparticles heat treated at 230 C. (Image from Teranishi, et. al. 2001. Advanced Materials 13 (22): 1699-1701. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Process:

1.Prepare DT-Au following Brust two-phase reaction procedure.
2.Add HAuCl4.4H20 (0.6 mmol) aqueous solution (60mL) to tetraoctylammonium bromide (TOAB, 1.2 mmol) in toluene (160 mL) and stir vigorously for 10 minutes at room temperature. The HAuCl4.4H20 is transferred into the organic layer turning orange and the yellow aqueous solution becomes colorless. Add to the mixed solution DT (0.6mmol) in toulene (20 mL) and stir for another 10 minutes at room temperature.
3.Add to the vigorously stirred solution fresh aqueous solution (60 mL) of sodium borohydride (6 mmol) -- the resulting solution turning black -- and stir for 12h.
4.Separate the organic phase via rotary evaporation at 40 degrees C and dry in vacuo at 30 degrees C for one day.
5.Heat-treat crude solid at 150, 190, and 230 degree C at the heating rate of 2 degrees C/min, and hold at this temperature for 30 minutes.
6.Dissolve heat-treated product in 30 mL toluene and mix with 300 mL methanol to remove excess DT and TOAB.
7.Filter off precipitate; wash with methanol; and redisperse in toluene.

Notes: For 250 degrees C anneal, replace dodecanethiol (DT) with octadecanethiol (ODT).

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